Chemical fertilizers are, as a rule, not fully utilized by the crop with much of the fertilizer material being lost to the soil environment and to the atmosphere. This occurrence is encountered practically with all fertilizers and is particularly pronounced with nitrogen-based fertilizers. The main reason for this deficiency is the very water-solubility of the fertilizer as a result of which undesired dissipation of the fertilizer by drainage and losses to the atmosphere in the form of NH.sub.3, N.sub.2 O, NO or NO.sub.2, compete with the root uptake thereof.
It has been suggested to overcome this problem by means of chemically prepared, slow-release fertilizers such as, for example, ureaform, isobutylidene diurea, oxamide and others, in which the fertilizer is present in form of a compound of reduced water-solubility. This method, however, has, among others, the disadvantage that it requires expensive raw materials and involves high manufacturing costs with the result that the price of the product may be prohibitive. Thus, for instance, in case of nitrogen-based fertilizers, the use of nitrogen compounds of reduced water solubility is at least three times as costly as the use of regular, water-soluble nitrogen fertilizers. Moreover, due to the so-called tailing effect, i.e. too slow decomposition, upon application to the soil, the efficiencies of chemically prepared fertilizers of reduced water solubility is often still unsatisfactory.
It is also known to control the dissolution rate of fertilizers by means of a physical barrier provided by coating of individual fertilizer particles with a composition of restricted water permeability or encapsulation of a cluster of fertilizer particles in matrices of such compositions, which composition will be referred to herein at times as "waterproofing material", and this type of slow-release fertilizer composition is capturing a steadily increasing market share. In the following, the terms "encapsulation" and "encapsulated" will be used in relation to slow-release particulate fertilizers of this kind, regardless of whether each particle of a particulate fertilizer is coated individually or whether clusters of such particles are encapsulated within matrices.
Encapsulated slow-release fertilizers may be classified into two major groups according to the fertilizer release mechanism:
(i) A first group in which the release is governed by the rate of water permeation through a polymeric or copolymeric membrane of the water-proofing material, and by the rate of fertilizer diffusion away from each coated particle into the surrounding soil. Typical examples of membrane material in slow-release fertilizers of this group are copolymers or glyceryl esters of unsaturated acids with dicyclopentadiene (U.S. Pat. No. 3,223,518), epoxy-polyester resins (U.S. Pat. No. 3,259,482), urethanes (U.S. Pat. No. 3,264,089) and polystyrenes (U.S. Pat. No. 3,158,462). The encapsulating polymer in slow-release fertilizers of this first group are generally applied either by spraying solutions of polymers dissolved in organic solvents as disclosed, for example, in U.S. Pat. No. 4,369,055, or by in situ polymerization which is brought about by the simultaneous spraying of several liquids onto the surface of the fertilizer particles as disclosed, for example, in U.S. Pat. Nos. 4,711,659 and 4,804,403. Either of these processes is, as a rule, carried out batchwise and generally large amounts of organic solvents are required which have to be recycled for economical reasons and also in order to avoid environmental pollution.
(ii) A second group with relatively thick encapsulating coats in which release is governed mainly by rupture of the coat, a typical example being particulate fertilizers with sulfur based encapsulation. The rupture occurs upon the permeation of water into the coated particles as a result of the osmotic pressure that builds up within. The sulfur based coating in slow-release fertilizers of this second group are generally produced by spraying onto the particulate fertilizer a molten sulfur based material as disclosed, for example, in U.S. Pat. No. 4,857,098. One major disadvantage of conventionally produced slow-release fertilizers with sulfur or sulfur based encapsulation is that very often the encapsulating coat is non-uniform, with the result that a significant amount of the fertilizer may be released within an unduly short period (so-called burst effect) due to the fact that the overlying coat is too thin, while the remainder of the fertilizer is retained within the encapsulated particles for an exceedingly long period due to excessive thickness of the overlying coat (so-called tailing or lock-off effect).
Further disadvantages of sulfur and sulfur based encapsulation produced by melt spraying are inefficient wetting of the fertilizer particles and bad adhesion and fragility of the encapsulating coat, all of which leads to peeling off of the coat from the fertilizer particles during transportation and handling.
Some improvement of sulfur based encapsulation may be achieved by applying to the encapsulating coat a further, external coating of wax, bitumen, synthetic resin and the like, followed, if desired, by spreading an inert powder on the wet external coating to reduce stickiness.
Quite generally, for successful performance of a melt spray, the coating material should possess a definite melting point, should have a low viscosity in the molten state, should solidify rapidly after application and yield a non-sticky encapsulating coat with good wetting and adhesion properties.
There are many amorphous materials, which, while capable of forming a good waterproofing film suitable to yield particulate fertilizer with slow-release properties, do not possess some of the above properties so that the production of slow-release fertilizers with such waterproofing materials is problematic.
AU 601,099 and AU Patent Application 89/29533 disclose a method for the production of particulate fertilizer which comprises wetting fertilizer particles with 1-5% by weight of water or an aqueous salt solution to serve as binder, followed by coating the so-wetted particles with a sulfur and/or a micronutrient powder. The final particulate product has no slow-release properties and the object of the powderous sulfur and/or micronutrient coating is merely to provide a readily disintegrating coat which upon application to the soil delivers finely divided sulfur and micronutrient particles, with the elementary sulfur being oxidized in the soil.
It is also known to apply mineral powder to a particulate fertilizer prior to the application of a waterproofing sulfur coating by way of a melt spray, and such a procedure is described, for example, in U.S. Pat. No. 3,576,613. According to this teaching, the fine powder particles adhere to the fertilizer and thereby facilitate subsequent even distribution of the sulfur melt on the entire granule surfaces.
It is further known in the art of drug coating (see, for example, "Controlled Release Systems: Fabrication Technology", edited by Dean S. T. Hsieh, v.1, CRC Press, Inc., Boca Raton, Fla., 1988, pp 109-142) that micro-capsules of soluble compounds with controlled release properties can be produced by a mechanochemical encapsulation process comprising blending the soluble compound to be protected in coarse particulate form, with a fine powderous waterproofing material. In this way, a powder layer is formed on the surface of each of the coarse soluble particles due to adhesion forces, e.g. of an electrostatic nature. After blending, the powder layers on the surfaces of the particles are filmed by prolonged mechanical agitation, preferably with heating. The coating obtained in this way is based on adhesion only and its thickness, uniformity and resistance to osmotic pressure are insufficient for making an encapsulated particulate fertilizer product with the desired long-term and constant release properties.
For the production of an encapsulated particulate fertilizer with good slow-release properties, it is essential that a uniform and coherent shell of waterproofing material adheres well to the fertilizer particles and it is the object of the present invention to provide a method for the production of encapsulated slow-release particulate fertilizer with good sustained and essentially constant release properties.
It is a further object of the present invention to provide an environmentally friendly method for the production of encapsulated slow-release particulate fertilizer products, which in its essential part does not require the use of organic solvents.